It is known that fumed silica is manufactured by flame hydrolysis, more specifically, by burning silicon chloride as a raw material in flame, and fumed silica is used in various fields such as reinforcers, abrasives, external additives, fillers and thickeners.
The fumed silica manufactured as described above is an agglomerate of silica having a primary particle diameter of 5 to 50 nm and very small in particle size and therefore difficult to handle. Particularly fumed silica which will become waste is difficult to handle because it is a fine particle and must be collected and disposed of not to contaminate work environment.
An example of fumed silica which is discharged as waste from the step of handling powdery fumed silica is fumed silica which remains in pipes, apparatuses and bags when it is packed or taken out from a bag. Since fumed silica is obtained from silicon chloride, hydrogen chloride is by-produced at the time of manufacturing fumed silica. Fumed silica which is not collected as a product is discharged together with hydrogen chloride.
Since particulate waste such as fumed silica may scatter during work to contaminate work environment as described above or contaminate the environment of its handling site, various studies on the handling and disposal of fumed silica are under way. For example, as means of disposing of fine powders, there are proposed methods of coagulating fine particles as a mass.
Stated more specifically, there are proposed a method in which fine particles to be swollen by hot water are kept in a vessel together with water and a high-temperature gas is injected to bind them together and coagulate them as a mass (see JP-A 6-71246), a method in which water, a fixation agent and a set accelerating agent are added to and mixed with fine powders (see JP-A 2001-276599), and a method in which a pH control agent or an oxidation agent is added to colloidal silica-containing drainage water and an inorganic flocculant is added to treat the drainage water (see JP-A 2004-261708).
However, since fine particles are coagulated to be treated in the first two methods, when the fine particles are coagulated, a coagulated product may adhere to a vessel or apparatus. Therefore, there is room for the improvement from a view point of operation efficiency. JP-A 2004-261708 is silent about the treatment of fumed silica-containing drainage water.
Further, JP-B 4-75796 discloses a flocculant for treating water, which is composed of a silica sol-metal salt. However, in the method disclosed by JP-B 4-75796, the treatment of only water having a given turbidity is carried out, and the method of treating fumed silica-containing drainage water which is the target of the present invention is not disclosed.